The Anterior Cruciate Ligament (ACL) in the human knee joint, commonly called the Cranial Cruciate Ligament in the canine stifle, is frequently torn in trauma or, as it happens in dogs, fails after a degenerative process of still unknown etiology. Direct repair is usually not possible; when attempted, it predictably fails.
In human orthopedics, the standard protocol calls for replacement by an autograft or an allograft, a part of the patellar ligament, or other tendons harvested for this purpose. The procedure results in a stable knee, but the long term performance is often unsatisfactory with over half of the cases resulting in arthrosis of the joint. There is also unwanted variability associated with the autograft and allograft tendons. Such natural tissue graft placement does not truly reproduce the function of the original ACL, as the grafts cannot perfectly imitate the anatomy of the original ACL.
In dogs, the standard procedure is either an extracapsular suture (usually placed on the lateral side of the joint) that approximates the function of the ligament or one of the geometry modifying surgical techniques (e.g. tibial plateau leveling osteotomy (TPLO), cranial closing wedge osteotomy (CWO), or tibial tuberosity advancement (TTA)). Intra-articular prostheses are also occasionally used, but those have generally failed. Extracapsular sutures also fail.
The canine techniques described above are in many cases intended to provide stability of the joint for several weeks while waiting for fibrosis to develop around the joint, which should then provide for long term stability. However, arthrosis of the joint is the rule rather than the exception within about one year. A variety of anchors have been used to fix the ends of an ACL reconstruction into bones. Most commonly, the anchors are so-called interference screws, which are designed to be inserted alongside the ligament replacement (e.g., a transplanted tendon or ligament or an artificial ligament) within an anchor hole or tunnel that is drilled into the bone. The interference screw jams the ligament replacement against the bone within the anchor hole. FIG. 1 shows an ACL bone anchor 1 of the interference screw type inserted, into a bone 3 in a direction 5 that jams the prosthetic tissue 2 within the hole 4 in order to resist a pull-out force 6. Such screws are made either from metal, most commonly titanium, or bioresorbable polymers.
In another commonly used technique, a so-called cross-pin technique, a loop of the prosthesis is anchored within a hole drilled in the femoral condyle. In all cases, the prosthesis exits the tunnel by bending over the edge of the bone. Healing and remodeling of the bone are expected to fill the gaps and to result in a natural-like anchorage of the prosthesis in the bone. FIG. 2 shows an ACL anchor 7 of the transverse, or cross-pin type, inserted into the bone of the femoral condyle 12 through a hole 10. A front section 8 of the anchor 7 passes through a loop of a prosthesis 9 inserted through the hole 11. In this manner, the prosthesis can support the pull 13 exerted on it in use.
Neither of these techniques is suitable for a permanent anchorage of an artificial material ACL replacement. Bending the prosthesis over the edge of a hole will lead to both bone loss due to contact resorption and mechanical damage of the ligament. Ultimately, the reconstruction fails due to bending fatigue and wear of the ligament and loss of bone at the edge of the hole.
An alternative method of using screw anchors with the prosthesis fixed in their central, axial hole also does not provide for a durable anchorage, because in all of the designs of which we are aware, the bending of the prosthesis over the edge of the hole exceeds the fatigue and abrasion limits of even the best materials.